0811: "Starlight"

This forum is for the individual discussion thread that goes with each new comic.

Moderators: Moderators General, Prelates, Magistrates

User avatar
jspenguin
Posts: 84
Joined: Wed Apr 16, 2008 7:39 pm UTC
Contact:

0811: "Starlight"

Postby jspenguin » Wed Oct 27, 2010 4:04 am UTC

Image
alt:"Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again."

Interesting question: when a photon hits a mirror, does the original photon bounce or does it get absorbed, then a new photon is emitted? Is there, physically speaking, a difference?

Code: Select all

from __future__ import skynet

Troger64
Posts: 48
Joined: Tue Oct 28, 2008 10:54 pm UTC

Re: 0811: "Starlight"

Postby Troger64 » Wed Oct 27, 2010 4:06 am UTC

I always assumed the photon bounced, but my major is in mathematics so I'm not exactly qualified to say...

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 4:07 am UTC

hmmm I think since photons are in constant interaction with the quantum vacuum, they can be replaced with identical virtual photons at any time. Idk?

User avatar
syko_lozz
Posts: 53
Joined: Fri Jan 11, 2008 5:30 am UTC
Location: Oz

Re: 0811: "Starlight"

Postby syko_lozz » Wed Oct 27, 2010 4:09 am UTC

surely it bounces? wouldn't absorption and reemission mean that mirrors were made out of stuff that can emit many different wavelengths of light? I dont think they're that special. But then again I'm a biology major, so who am I to comment?
Debate politics with a fern. If you lose, refuse to water it.

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 4:11 am UTC

I think the photon interacts with the electron, bringing it to a higher energy state, then the electron falls back and re-emits it. The difference probably doesn't matter

Meem1029
Posts: 379
Joined: Wed Jul 21, 2010 1:11 am UTC

Re: 0811: "Starlight"

Postby Meem1029 » Wed Oct 27, 2010 4:12 am UTC

That's an interesting question. As a physics major I will eventually be able to say probably, but as a freshman I have no idea. I guess I have always thought of it as just bouncing with the same photon, but I'm not sure. As far as a difference, I think there is some sort of a difference because of the concept of quantum cryptography making it so listening can be detected, but I don't know. Of course thinking about it if the photon always moves at c it would have to have infinite acceleration to turn around completely and instantly. That or I just don't really understand what goes on at those levels (OK, I know I don't fully understand it, but I think I have a bit of a clue).

Edit: Reading through the alt-text again made me realize something I want to confirm. Because of relativity, wouldn't a photon technically only exist for an instant (or actually no time) in it's own reference frame? Or would it be forever? Asking random questions about relativity is so much more interesting than the lab I'm supposed to be doing.
Last edited by Meem1029 on Wed Oct 27, 2010 4:17 am UTC, edited 1 time in total.
cjmcjmcjmcjm wrote:If it can't be done in an 80x24 terminal, it's not worth doing

User avatar
madock345
Posts: 51
Joined: Mon Apr 12, 2010 12:26 am UTC
Location: My used book store near Seattle WA

Re: 0811: "Starlight"

Postby madock345 » Wed Oct 27, 2010 4:13 am UTC

Science is fun but i have to say
Dawwwww
beret-guy is soooo cute remindes me of the one about his mom
"As the size of an Explosion increses the number of social situations it is incapable of resolving approches zero"
-Vaarsuvius, The Order of the Stick
Spoiler:
ethereal_fire wrote:madock345, I like your sig : )

Qwerty.55 wrote:I like your new recursive sig even better than before.

User avatar
glasnt
Posts: 539
Joined: Fri Jan 25, 2008 5:18 am UTC
Location: SQUEE!

Re: 0811: "Starlight"

Postby glasnt » Wed Oct 27, 2010 4:18 am UTC

HI JOEE

Beret guy is cute.

I would suggest that solarpanels could be a happy reflecty bed, I'm sure that could be marketed somehow.

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 4:20 am UTC

Meem1029 wrote:Of course thinking about it if the photon always moves at c it would have to have infinite acceleration to turn around completely and instantly. That or I just don't really understand what goes on at those levels (OK, I know I don't fully understand it, but I think I have a bit of a clue).

As far as I understand (high school senior with a physics obsession) spacetime is quantized too so infinities don't occur, and newtonian mechanics are not accurate at that level. Aaaaaand newtonian physics doesn't describe entities with no rest mass.

Faranya
Posts: 259
Joined: Tue Nov 18, 2008 3:10 am UTC

Re: 0811: "Starlight"

Postby Faranya » Wed Oct 27, 2010 4:29 am UTC

I doubt that bouncing is the solution to this. I would hypothesize that the photon is absorbed by the material, and then the material of the mirror emits another photon of approximately the same, but slightly less energy.
Image

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 4:31 am UTC

Faranya wrote:I doubt that bouncing is the solution to this. I would hypothesize that the photon is absorbed by the material, and then the material of the mirror emits another photon of approximately the same, but slightly less energy.

At that scale there is nowhere for the energy to go, so the photon would be basically identical in energy (frequency)

User avatar
Sam Knight
Posts: 68
Joined: Mon Nov 05, 2007 5:30 am UTC
Location: A fighter jet made of BICEPS!

Re: 0811: "Starlight"

Postby Sam Knight » Wed Oct 27, 2010 4:32 am UTC

I don't get this comic at all.. Why the mirror?
Image

User avatar
Me321
Posts: 164
Joined: Fri Oct 09, 2009 4:03 am UTC

Re: 0811: "Starlight"

Postby Me321 » Wed Oct 27, 2010 4:35 am UTC

Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 4:37 am UTC

Sam Knight wrote:I don't get this comic at all.. Why the mirror?

He wants the photons to be able to travel back home. He probably should have used three mirrors at right angles (corner retro-reflector) so they would actually reflect straight back at their source ;)

User avatar
LucasBrown
Posts: 299
Joined: Thu Apr 15, 2010 2:57 am UTC
Location: Poway, CA

Re: 0811: "Starlight"

Postby LucasBrown » Wed Oct 27, 2010 4:40 am UTC

Ruined by the title-text. I like the comic and I like the title-text, but the combination of the two is awful.

Kaijyuu
Posts: 101
Joined: Tue Oct 14, 2008 1:58 am UTC

Re: 0811: "Starlight"

Postby Kaijyuu » Wed Oct 27, 2010 4:45 am UTC

Relativity saves the day again. Yay!

Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

You'd need a black hole to do that. Plus the vast majority of the photons would get sucked in, which is a fate far worse than being absorbed by your eye.
Last edited by Kaijyuu on Wed Oct 27, 2010 4:49 am UTC, edited 2 times in total.
The cake is a lie, but truth is in Pi.

User avatar
joee
Posts: 227
Joined: Mon Dec 03, 2007 5:53 am UTC

Re: 0811: "Starlight"

Postby joee » Wed Oct 27, 2010 4:46 am UTC

I interpreted that as a photo frame and thought it was van gogh going "okay, which section of sky am I going to paint"

HI GLASNT
Hi glasnt.

Singulaire
Posts: 37
Joined: Fri May 14, 2010 10:05 am UTC

Re: 0811: "Starlight"

Postby Singulaire » Wed Oct 27, 2010 4:46 am UTC

I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

Meem1029
Posts: 379
Joined: Wed Jul 21, 2010 1:11 am UTC

Re: 0811: "Starlight"

Postby Meem1029 » Wed Oct 27, 2010 4:51 am UTC

If the effect does come because of absorption, why does the light always reflect back with equal angles of incidence. Actually, why does it either way? On a macroscopic scale it seems logical that it would, but when looking on an atomic scale there doesn't seem to be any good reason for this.

Also, I wonder if scientists actually know the answer to these questions or if they are open questions that are just mostly irrelevant.
cjmcjmcjmcjm wrote:If it can't be done in an 80x24 terminal, it's not worth doing

Mapar
Posts: 129
Joined: Wed Jun 16, 2010 11:26 am UTC

Re: 0811: "Starlight"

Postby Mapar » Wed Oct 27, 2010 4:59 am UTC

The photon is absorbed, I read that about a year ago in a scientific article. It was in Knack, one of our local "weeklies".
Hi.

User avatar
BlazeOrangeDeer
Posts: 61
Joined: Tue Aug 25, 2009 3:44 am UTC

Re: 0811: "Starlight"

Postby BlazeOrangeDeer » Wed Oct 27, 2010 5:02 am UTC

Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

I thought thermodynamics were a macro effect of quantum events, just as newtonian mechanics are basically the total of quantum events in a large system. So yeah, they basically "go wonky" at the quantum scale and the photon retains the energy.

Meem1029
Posts: 379
Joined: Wed Jul 21, 2010 1:11 am UTC

Re: 0811: "Starlight"

Postby Meem1029 » Wed Oct 27, 2010 5:03 am UTC

Do you have a link or title for that, because that would be a very interesting article to read?

(O.T., but does it seem awkward to anyone else when you ask a question with a side-note at the end as part of the sentence because the punctuation just seems bizarre for that? And I just realized that I did the same thing in my question right there.)
cjmcjmcjmcjm wrote:If it can't be done in an 80x24 terminal, it's not worth doing

Arariel
Posts: 404
Joined: Fri Sep 17, 2010 2:32 am UTC

Re: 0811: "Starlight"

Postby Arariel » Wed Oct 27, 2010 5:06 am UTC

Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

But why lower energy (and thus frequency)? I don't think light gets any redder when it hits a mirror. Could it be intensity instead?

Airbuilder7
Posts: 21
Joined: Mon Dec 14, 2009 6:20 am UTC

Re: 0811: "Starlight"

Postby Airbuilder7 » Wed Oct 27, 2010 5:09 am UTC

I assumed that the rectangular object was a picture frame, in order to refute the woman's cynicism with the beauty of the stars... But now that I look at it, there seem to be specks in the center of the rectangle when he's carrying it, so it might be a mirror as some have suggested. I don't know; I lean toward the poetic picture frame. After all, if this particular starlight did not end its journey in our eyes, it would fly past our small planet and travel onward in the black for another 14 billion years, never to be known and appreciated.

This comic reminds me strongly of a YouTube video about the Hubble Ultra Deep Field I came across last year. The girl's same argument could be made about the photons finally coming to rest against the image collecting plate. Amazing stuff.

EDIT: Found the link. http://www.youtube.com/watch?v=oAVjF_7ensg

Maybe it's the music, but the sheer implications of the image make me tear up a little. Go science. Go mankind. :)
Last edited by Airbuilder7 on Wed Oct 27, 2010 5:21 am UTC, edited 1 time in total.

User avatar
Time Kitten
Posts: 28
Joined: Mon Jul 19, 2010 4:38 am UTC
Location: Behind you!
Contact:

Re: 0811: "Starlight"

Postby Time Kitten » Wed Oct 27, 2010 5:12 am UTC

Hmm... I just figured that if anything shiny is that way because of the electron sea thingy that also makes it conductive and everything, and traditionally to bounce photons you use electron streams, it'd be bouncing off of the reflective surface... No clue how glass works, still.

Subject17
Posts: 2
Joined: Thu Sep 16, 2010 12:14 am UTC

Re: 0811: "Starlight"

Postby Subject17 » Wed Oct 27, 2010 5:13 am UTC

I was under the impression that atoms could only emit photons at a certain wavelength/frequency, hence emission spectra. Since mirrors can reflect all of visible light (along with infrared and ultraviolet) essentially "perfectly" (barring impurities, etc) back, or at least nearly true to the photon's original energy, wouldn't that mean they have to bounce? Isn't it similar to how colors work (ie blue reflects blue/absorbs all others, etc), or was that just a simplification to the real effects?

Ugghh. We need someone on a grad level of physics before I start to trust any information suggested here.

Rilian
Posts: 496
Joined: Mon Sep 10, 2007 1:33 pm UTC

Re: 0811: "Starlight"

Postby Rilian » Wed Oct 27, 2010 5:14 am UTC

Why does it say the light dies here?
And I'm -2.

User avatar
Eternal Density
Posts: 5590
Joined: Thu Oct 02, 2008 12:37 am UTC
Contact:

Re: 0811: "Starlight"

Postby Eternal Density » Wed Oct 27, 2010 5:15 am UTC

Pi Cheers for Relativity!
(Or just 3 if you want to keep things rational.)
Arariel wrote:
Singulaire wrote:I've actually discussed that question with my high school physics teacher and a couple of friends (hurray for end-of-the-year lessons when you're not really studying anymore). Our final verdict was that- barring some unknown quantum effect that makes thermodynamics go wonky at that level- the photon should be absorbed, excite an atom, and as the electron returns to its base orbit, a slightly less energetic photon will be emitted.
Ofcourse, despite my use of the word "verdict", a bunch of student and a high school teacher, addorned by whatever degree they may be, are by no means an authority.

But why lower energy (and thus frequency)? I don't think light gets any redder when it hits a mirror. Could it be intensity instead?
Yeah, the electron is going to fall back to the energy level it started from, thus the same colour of light. When looking at an 'infinite' reflection between parallel mirrors, it gets darker, not redder.
In other words, HighSchoolPhysicsTeacherFail.
Play the game of Time! castle.chirpingmustard.com Hotdog Vending Supplier But what is this?
In the Marvel vs. DC film-making war, we're all winners.

Retsam
Posts: 57
Joined: Tue Aug 18, 2009 5:29 am UTC

Re: 0811: "Starlight"

Postby Retsam » Wed Oct 27, 2010 5:16 am UTC

Definitely a GOOMMR moment. I was trying to procrastinate my astronomy essay by reading xkcd... darnit.

leifbk
Posts: 33
Joined: Wed Nov 26, 2008 7:24 am UTC
Location: Bærum, Norway
Contact:

Re: 0811: "Starlight"

Postby leifbk » Wed Oct 27, 2010 5:16 am UTC

I believe that reflection happens to the light in its wave-form. That said, that dual nature of light has always bothered me.

User avatar
big boss
Posts: 589
Joined: Tue Mar 23, 2010 7:59 am UTC

Re: 0811: "Starlight"

Postby big boss » Wed Oct 27, 2010 5:20 am UTC

If the mirror absorbed and re-emitted the photons there is no reason why they should all be re-emitted in a coherent way to recreate the incident image. So i dont think that that is a valid explanation. Also the thermal motions of the atoms in the mirror would make recreating the originally image after reflection impossible because thermal vibrations would distort the paths of the reflected photons.

Edit: If one looks at the mirror as a high potential barrier (a Dirac delta function possibly?) in the path of the wavefunction of the photons. When the wavefunction of the photons hits this barrier they have a nonzero probability of reflecting and going in the opposite direction (this is not to say that they were absorbed by the mirror, they were just reflected).
Last edited by big boss on Wed Oct 27, 2010 5:24 am UTC, edited 1 time in total.
"Starbuck, what do you hear?"
"Nothing but the Rain."

rpgamer
Posts: 109
Joined: Thu Oct 01, 2009 3:54 am UTC

Re: 0811: "Starlight"

Postby rpgamer » Wed Oct 27, 2010 5:22 am UTC

Me321 wrote:Then use gravity to bend it arround the planet, you just need a very massive object at the correct location.

Is this the setup for a "Your mom!" joke?

To other posts: I also thought the object was a picture frame at first. It kinda works both ways, the artistic beauty of those pretty dots viewed in frame, or trying to send the starlight back home.

On the subject of starlight, I've always been intrigued by the idea that everything we see out there happened so long ago. Some of it maybe not so long ago, but there's still plenty of pre-Earth light finally making it here. And also the whole space expansion, and how beyond a certain point, light out there will never reach us, with space expanding away faster and faster and so forth.
Last edited by rpgamer on Wed Oct 27, 2010 5:32 am UTC, edited 1 time in total.
All it takes is one bad day to reduce the sanest man alive to lunacy. That's how far the world is from where I am. Just one bad day.

Ego
Posts: 22
Joined: Tue Sep 22, 2009 2:29 am UTC

Re: 0811: "Starlight"

Postby Ego » Wed Oct 27, 2010 5:27 am UTC

Mirror doesn't absorb-and-then-emit photones. It seems like photons are bounced from free electrons in silver surface of mirror.
http://en.wikipedia.org/wiki/Metallic_b ... properties
Yeah, mining physics knowledge from wikipedia is lame.

Singulaire
Posts: 37
Joined: Fri May 14, 2010 10:05 am UTC

Re: 0811: "Starlight"

Postby Singulaire » Wed Oct 27, 2010 5:27 am UTC

wait, why are we talking about intensity? I'm pretty sure that's a result of the same number of photons being spread over a larger area, which doesn't work with a single photon.
Also, a mirror reflectd all wavelengths? hmm, maybe that would apply to all wavelengths in a certain range, but if my mirror can reflect X-rays, that would lend itself to some interesting pranks. Maybe I'm just getting confused because we're using different definitions of "mirror".
Also, if anyone could provide an article about thermodynamics not working at such a microscopic level, I would love to read it. That was pretty much the thing that stumped us actually- we couldn't agree whether energy could or couldn't be lost in the interaction.

Edit: Ego, I love you.

Second Edit: To elucidate, the discussion was about whether photons lose energy when interacting with atoms (even if infinitsimally small) as part of a larger discussion that can be described as "What happens when light travels, and other interesting minutiae".
Last edited by Singulaire on Wed Oct 27, 2010 5:39 am UTC, edited 2 times in total.

rcox1
Posts: 112
Joined: Wed Nov 11, 2009 11:23 pm UTC

Re: 0811: "Starlight"

Postby rcox1 » Wed Oct 27, 2010 5:31 am UTC

Very cute comic.

As far as the rectangular thing, I think it is a mirror to reflect the light back into space.

As far the pedants, the hypothetical individual photon has probably interacted with many atoms in it's trip to your eye, not the least of which is all the gunk we call the atmosphere. It has been absorbed and reemitted many times so it is a philosophical argument.

To be pseudo philosophical, the starlight is composed many photons which we can wave our hands and say that it forms some sort of wave function(ugh, that has to be wrong), and that the density decreasing cubically as the it travels, which means by the time it hits only very few photons of the star will hit the earth, while most will travel through empty space, unhindered. As such the perturbation to the original function would be minimal. All this, of course, is complete crap.

What the comic did remind of is the Feynman one electron in the universe notion, and by extension one photon, up, down, top, bottom, charm and strange quark, etc. When I took my QM course and perused the lecture notes and popular work, this along with the diagrams seemed a very elegant way to look at the universe.

ijuin
Posts: 1152
Joined: Fri Jan 09, 2009 6:02 pm UTC

Re: 0811: "Starlight"

Postby ijuin » Wed Oct 27, 2010 5:34 am UTC

For elections that are bound to individual atoms, that would be true (and it would be why non-reflective materials have colors instead of being all white or silver). However, the "electron sea" in metals is not bound to orbitals, and thus can absorb/release a much wider band of energies.

AFAIK the whole "reflects at the same angle" part is an example of the dual particle/wave nature of photons--it's a wavelike property.

coder0xff
Posts: 1
Joined: Wed Oct 27, 2010 5:39 am UTC

Re: 0811: "Starlight"

Postby coder0xff » Wed Oct 27, 2010 5:43 am UTC

"In the classical electrodynamics, light is considered as electromagnetic wave, which is governed by the Maxwell Equations. Light waves incident on a material induce small oscillations of polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave (in all directions, like a dipole antenna). All these waves add up to give specular reflection and refraction, according to the Huygens-Fresnel principle." http://en.wikipedia.org/wiki/Reflection_(physics)#Mechanisms_of_reflection

That said, this comic made me go, "awwww." :D

User avatar
Sam Knight
Posts: 68
Joined: Mon Nov 05, 2007 5:30 am UTC
Location: A fighter jet made of BICEPS!

Re: 0811: "Starlight"

Postby Sam Knight » Wed Oct 27, 2010 5:45 am UTC

BlazeOrangeDeer wrote:
Sam Knight wrote:I don't get this comic at all.. Why the mirror?

He wants the photons to be able to travel back home. He probably should have used three mirrors at right angles (corner retro-reflector) so they would actually reflect straight back at their source ;)


Haha, trying to quickly cram today's XKCD comic in while working on my neuro paper = me thinking the people had travelled trillions of miles to die under the starlight, and so mirror.
Image

Hypoon
Posts: 9
Joined: Wed Oct 27, 2010 5:25 am UTC

Re: 0811: "Starlight"

Postby Hypoon » Wed Oct 27, 2010 5:45 am UTC

Wikipedia seems to have an intelligent section on this: http://en.wikipedia.org/wiki/Reflection_%28physics%29
Mechanisms of reflection

In the classical electrodynamics, light is considered as electromagnetic wave, which is governed by the Maxwell Equations. Light waves incident on a material induce small oscillations of polarisation in the individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave (in all directions, like a dipole antenna). All these waves add up to give specular reflection and refraction, according to the Huygens-Fresnel principle.

In case of dielectric (glass), the electric field of the light acts on the electrons in the glass, the moving electrons generate a field and become a new radiator. The refraction light in the glass is the combined of the forward radiation of the electrons and the incident light and; the backward radiation is the one we see reflected from the surface of transparent materials, this radiation comes from everywhere in the glass, but it turns out that the total effect is equivalent to a reflection from the surface.

In metals, the electrons with no binding energy are called free electrons. The density number of the free electrons is very large. When these electrons oscillate with the incident light, the phase differences between the radiation field of these electrons and the incident field are π, so the forward radiation will compensate the incident light at a skin depth, and backward radiation is just the reflected light.

Light–matter interaction in terms of photons is a topic of quantum electrodynamics, and is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter.


More corroborating evidence from Scientific American: http://www.scientificamerican.com/article.cfm?id=how-do-mirrors-reflect-ph
To understand how such mirrors work, let us first describe the interaction of light with some media in the semiclassical view. Light consists of electromagnetic waves, which induce some oscillation of electrons in any substance hit by the light. In an insulator such as glass, the electrons are firmly bound and can only oscillate around their normal position. This movement influences the propagation of light so that its wave velocity is reduced, while there is only a small loss of energy. This is different in a metal, where some of the electrons are free to move over large distances, but their motion is damped so that energy is dissipated. The wave amplitude decays very quickly in the metal--usually within a small fraction of the wavelength. Associated with that decay is a loss of energy in the wave and some heating of the metal. Most of the incident optical power, however, is reflected at the air/metal interface. In other words, the power is transferred to another wave with a different propagation direction (opposite to the original direction for normal incidence on the surface).


The what I've learned while getting my undergraduate physics degree so far supports the above. Applying conservation of momentum to the particle nature of light will show that the angle of reflection must be the same as the incident angle (on the other side of the normal, of course). The wave nature of light covers most other aspects.

fow
Posts: 6
Joined: Wed Apr 09, 2008 3:11 am UTC

Re: 0811: "Starlight"

Postby fow » Wed Oct 27, 2010 5:56 am UTC

Don't worry! From the light's point of view, home and your eye are in the same place, and the journey takes no time at all! Relativity saves the day again.


Actually, from the light's point of view, everything is in one of two places. Everything behind it in one, everything in front in the other.

So your eye and home are as far apart as they can be. So, so sad.


Return to “Individual XKCD Comic Threads”

Who is online

Users browsing this forum: sotanaht and 92 guests